In many communications and data processing applications it is necessary to accurately detect the peaks of an input voltage signal. For example, the process of reading digital data written onto a recording medium, such as a disc, results in an alternating read signal in which the information is contained in the phases of the signal peaks. The information is recovered by applying the read signal to a peak detecting network that provides a sequence of indications, typically in the form of logic level transitions, corresponding in time to the peaks of the input read signal.
The actual detection of the peaks of an input signal is frequently accomplished by translating these peaks into corresponding zero crossings of a peak detector signal, and thereafter sensing the occurrence of the resulting zero crossings by means of a comparator network. The translation of the input signal peaks into zero crossings of the peak detector signal is typically accomplished by applying a 90.degree. phase shift to the input signal by either signal integration or signal differentiation.
The most commonly used approach to peak detection has been to differentiate an input signal to obtain a differentiated signal having zero crossings corresponding to the peaks of the input signal. Because the differentiation process provides an output that corresponds at each instance of time to the differential of the input signal, differentiating peak detectors have generally been susceptible to high frequency noise and transient voltage spikes. In particular, the spectral response of the differentiator is such that the differentiator amplifies high frequency noise more than the signal. Also, the amplitudes of transient voltage spikes are often comparable to the amplitude of the input signal peaks that must be detected, thus making detection difficult and unreliable. As a result, differentiating peak detectors must commonly employ fairly complex noise rejection circuitry in order to reliably detect the peaks of the input signal.
It has been suggested that integrating peak detectors be used in order to avoid the noise susceptability of differentiating peak detectors. The integration process is inherently less noisy because it looks at the energy in a signal over a period of time, and not just at an instantaneous differential value. Thus, integration can be used to substantially reduce the effect of high frequency noise, since, even though the amplitude of the noise may be large, the integrated value of the noise over the period of the lower frequency input signal averages out to be extremely small. In other words, the spectral response of an integrator is much more band width-limited than that of a differentiator.
Notwithstanding the inherent noise rejection properties of integration, currently available integrating peak detectors do have a number of disadvantages. First, integrators produce a slowly varying DC component as part of their output waveforms. If the integrating peak detector merely compares the integrator signal to a fixed ground or reference level, then peak detection is susceptible to errors caused by a sufficiently large DC component of the integrator signal. Additional errors may result because the slowly varing DC component causes some peaks to be detected earlier or later in time than their actual time of occurrence. The latter situation is particularly unacceptable in those applications, such as the data processing application mentioned above, in which the accurate retrieval of the information contained in an input signal depends upon preserving the precise peak phase relationships of the input signal. Accordingly, in order to take advantage of the inherent noise rejection properties of an integration approach to peak detection, it has been heretofore necessary to incorporate circuitry designed to counteract the problem of a slowly varying DC component in the output of a peak detecting integrator.
It is a general object of the present invention to provide a relatively uncomplicated network to detect the occurrence and sequence of the peaks of an input signal. To this end, it is a specific object of the present invention to provide a peak detecting network exhibiting a high degree of immunity to superimposed high frequency noise and noise voltage spikes.
It is a further object of the present invention to provide such a peak detecting network that substantially avoids the build-up of a DC component and that exhibits a high degree of immunity to low freqency noise without having to incorporate additional circuitry expressly for this purpose.
It is another object of the present invention to provide such a peak detecting network utilizing a passive filter network rather than active differentiation or integration circuitry.